The convergence or divergence of an optical beam in a traditional, refraction-based device depends on the phase change of the light propagating inside the device. The strength of the light bending in such a system is therefore limited by the refractive index of a given dielectric. Fabrication challenges are also paramount, as it is very difficult to make devices with a large aperture and a short focal length. By using the Fresnel lens design, the mass and volume of material can be reduced, but the thickness of the device is still on the wavelength scale. Fresnel zone plates, which consist of concentric rings (Fresnel zones) and use diffraction instead of refraction or reflection, also can be used to focus or diffract light, but it is impossible to shrink the size down to only a few wavelengths since the radius differences between the neighboring opaque and transparent rings must be at least half of the wavelength of the incident light, and typically a large number of rings is required for good performance.
Advances in the area of plasmonics have now opened up a new era for building compact, planar lenses. A number of plasmonic lenses have been developed recently based on super-oscillation and mode-index manipulation of guided waves inside nano-apertures (i.e. slits or holes). Nevertheless, those designs suffer from limited phase control, which restricts their minimum sizes and thicknesses: either the size of the device cannot be further reduced because the design is based on the diffraction of the light through transparent/opaque regions, or the thickness of the device must be comparable to the operational wavelength because the phase change is obtained by light propagating inside the lens material.
In the last few years, subwavelength-sized plasmonic nano-antennas on a planar surface have been shown to create phase shifts covering the full range (from 0 to 2π) in cross-polarized scattered light due to their asymmetric plasmonic resonances. An array of such nanoantennas can form a metasurface to bend the light abnormally in a fairly broad range of wavelengths and can create, for example, an optical vortex beam. In addition, a metasurface arranged of plasmonic nano-antennas can be used as a very efficient coupler between propagating waves and surface waves. The design of these phase-shifting, plasmonic nano-antennas also can be used to build optical devices, including but not limited to lenses, with surprising properties.